Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Brush fish with lemon-juice. Sprinkle desired seasoning evenly over all sides of fish. For sandwich spread, in a small bowl stir together mayonnaise, mustard, and honey. Cover and chill until ready to serve.
Place fish in a greased grill basket, tucking under any thin edges. For a charcoal grill, grill fish on the rack of an uncovered grill directly over medium coals until fish flakes easily when tested with a fork (allow 4 to 6 minutes per 1/2-inch thickness of fish), turning basket once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place fish on grill rack over heat. Cover and grill as above.)
Remove fish from grill. To serve, spread bottom half of buns with mayonnaise mixture. Top with watercress (if desired), fish, additional mayonnaise mixture, red pepper strips (if desired), and top halves of buns. Makes 4 servings.